Surfactant–Ligand Design for ab Initio Emulsion Atom Transfer

Article pubs.acs.org/Macromolecules

Surfactant−Ligand Design for ab Initio Emulsion Atom Transfer Radical Polymerization Yipeng Wei,† Yanyu Jia,† Wen-Jun Wang,*,†,‡ Bo-Geng Li,† and Shiping Zhu*,§ †

State Key Laboratory of Chemical Engineering, College of Chemical and Biological Engineering, and ‡Key Lab of Biomass Chemical Engineering of Ministry of Education, College of Chemical and Biological Engineering, Zhejiang University, Hangzhou, Zhejiang 310027, China § Department of Chemical Engineering, McMaster University, Hamilton, Ontario L8S 4L7, Canada S Supporting Information *

ABSTRACT: We report an ab initio emulsion atom transfer radical polymerization (ATRP) based on micellar nucleation. A surfactant−ligand (SL) was designed and synthesized for this purpose. The SL design was an amphiphilic macromolecule, and it acted as the surfactant for micelle formation and as the ligand for ATRP catalyst. The micelles were converted into to polymer particles with both water-soluble and oil-soluble initiators. The micellar nucleation mechanism was confirmed by the growth of polymer particles during polymerization, in comparison to a control experiment without SL addition. It was also found that diffusion-controlled deactivation inside particles occurred, which broadened the polymer molecular weight distribution. Adding a small amount of free catalyst significantly improved the molecular weight control. The polymers resulting from the ab initio emulsion ATRP were proven living by chain extension experiments.

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The first work on emulsion ATRP employed the Cu(II)/bpy catalyst system.17,18 The polymerization was fast and yielded high-MW polymers but broad distribution with PDI > 2. It was attributed to departure of catalytic species from polymer particles. More hydrophobic ligands such as dNbpy and BPMODA were then used to improve retaining of catalyst in oil.19−22 With these ligands, good molecular weight control was achieved. However, a careful examination revealed that these systems were actually not ab initio emulsion ATRP. Highly hydrophobic long-chain alkyls resided in monomer droplets and prevented monomer molecules migrate to particles due to osmotic pressure. The monomer droplets became major sites for polymerization. Such systems were more or less like miniemulsion ATRP or so-called aqueous dispersed ATRP, but not the originally attempted ab initio emulsion polymerization.14,23 Using seeded emulsion technology,23,24 Matyjaszewski et 23 al. encapsulated catalyst complexes in polymer particles prior to monomer addition. The monomer droplets were thus free of highly hydrophobic ligand molecules and acted as reservoir in supplying monomer for growth of seeded polymer particles. This was probably the first truly emulsion polymerization based on ATRP mechanism. But it was a seeded emulsion ATRP. The objective of this work is to design a one-step ab initio emulsion ATRP system based on micellar nucleation. The

INTRODUCTION Controlled radical polymerization (CRP), such as atom transfer radical polymerization (ATRP),1 nitroxide-mediated radical polymerization (NMP),2 and reversible addition−fragmentation chain transfer radical polymerization (RAFT),3 provides a powerful tool for preparation of polymers having wellcontrolled chain microstructures and topologies. Over the past two decades, numerous investigations have been reported on solution CRP systems,4−7 with limited commercial successes. In comparison, emulsion CRP received much less attention, regardless of its clear advantage in organic solvent elimination and energy saving in polymer separation, as well as high polymerization rate and high polymer molecular weight. Emulsion polymerization, however, is inevitably more complex than its solution counterpart. A review of literatures reveals that most studies focused on miniemulsion systems,8−16 which in general has little potential in commercialization because of their great energy assumption required in emulsification and large amount of emulsifier in particle stabilization. On the other side, emulsion CRP does not have the same problems and remains to be the most promising process. Emulsion ATRP has been particularly challenging with unstable latex and uncontrollable polymer molecular weight. There are several major issues yet to be resolved. One is the partitioning of catalyst complexes between oil and water phases. For a successful emulsion ATRP, there must be adequate Cu(II) concentration in oil. Unfortunately, Cu(II) complex tends to leave oil, leading to unstable latex and uncontrolled molecular weight as well as aqueous phase pollution. © 2014 American Chemical Society

Received: September 14, 2014 Revised: October 23, 2014 Published: November 5, 2014 7701

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(Unico 2802). Ultrafiltration tubes (MW = 3K, 15 mL) were purchased from Millipore. The particle size of the latex was determined by dynamic light scattering (DLS) (Malvern, ZS-90) where samples were diluted before the testing. Synthesis of Surfactant−Ligand (SL). In step I, mPEG was dried in a vacuum oven at 70 °C for 4 h. Then, mPEG (10 g, 0.01 mol) and TEA (2.1 mL, 0.015 mol) were dissolved in 100 mL of THF. The mixture was added to a 250 mL flask, which was placed in an ice− water bath. 10-Undecylenoyl chloride (3.2 mL, 0.015 mol) was added dropwise to the solution over a 15 min period under stirring. When the addition was completed, the reaction was carried out at 25 °C for 24 h. The solution was then filtrated to remove impurities and then precipitated in n-pentane for three times. The final product was dried under vacuum for 24 h, yielding a white solid. Yield = 88%. 1H NMR (400 MHz, D2O): δ = 5.80 (1H, CH), 4.86 (2H, CH2), 4.13 (2H, −CH2OCO), 3.26 (3H, CH3−), 2.25 (2H, OCOCH2−). 13C NMR (400 MHz, δ, D2O): 174.34, 138.79, 114.3, 71.08−69.80, 63.63, 58.13, 33.77, 29.09, 24.74. In step II, the product (5 g, 0.004 mol) from step I and mCPBA (1.22 g, 0.006 mol) were dissolved in 25 mL of DMC to start the reaction, which proceeded at 25 °C for 24 h. Afterward, the solution was filtrated to remove insoluble byproduct, and then DMC was distilled under reduced pressure. This solid was dissolved in 30 mL of THF, and the mixture was passed through a basic alumina column with THF as the eluent. The crude product was purified by dialysis for 2 weeks, yielding a white solid. Yield = 60%. 1H NMR (400 MHz, D2O): δ = 4.15 (2H, −CH2OCO), 3.26 (3H, CH3−), 3.00 (1H, ), 2.79 (1H, ), 2.54 (1H, ), 2.35 (2H, OCOCH2−). 13C NMR (400 MHz, δ, D2O): 176.72, 71.08−68.57, 65.56, 60.42, 58.11, 54.00, 48.03, 33.89−31.33, 29.00−28.58, 25.34−24.84. In step III, the product (2.36 g, 0.002 mol) from step II and TEDETA (5.00 g, 0.02 mol) were dissolved in 10 mL of IPA. The mixture was added to a 100 mL flask and reacted at 85 °C for 10 h under the N2 atmosphere. The yellow solution was precipitated using cold n-pentane. The product was dried under vacuum for 24 h, yielding a yellow solid. Yield = 90%. 1H NMR (400 MHz, CDCl3): 4.20 (7.8 Hz, 2H, −CH2OCO), 3.35 (3H, CH3−), 2.58 (16H, NCH2), ). 13C NMR (400 MHz, δ, 2.29 (2H, OCOCH2−), 1.03 (12H, D2O): 176.56, 71.78−68.57, 63.78, 60.42−58.12, 50.79−46.89, 37.77, 34.49−33.89, 29.00−28.64, 25.98−24.88, 10.00−9.25. General Procedure for Emulsion ATRP. In the emulsion ATRP with water-soluble initiator, take run 2 as example, CuCl2 (0.0134 g, 0.1 mmol), SL (0.133 g, 0.95 mmol), and dNbpy (0.002 g, 0.05 mmol) were dissolved in BMA (1.42 g, 0.01 mol) to form a homogeneous solution (oil phase). Brij98 (0.08 g) was dissolved in deionized water (12 g) to form aqueous phase. The oil phase was then added to the aqueous phase in a flask under stirring. The flask was degassed with nitrogen for 1 h. VA-044 (0.0484 g, 0.15 mmol) was dissolved in deoxygenated water (1 g). The flask was placed in an oil bath at 80 °C. VA-044 solution was finally added to start the polymerization. Samples were taken at preset time intervals. In the emulsion ATRP with oil-soluble initiator, the procedure was similar to that with water-soluble initiator. Take run 6 as example, CuCl2 (0.0067 g, 0.05 mmol), EBiB (0.0097 g, 0.05 mmol), SL (0.066 g, 0.0475 mmol), and dNbpy (0.0019 g, 0.0025 mmol) were dissolved in BMA (1.42 g, 0.01 mol). Tween80 (0.08 g) was dissolved in deionized water (12 g). The aqueous and oil phases were added to a flask with stirring to form an emulsion, following by degassing. The flask was placed in an oil bath set to 70 °C. AA (0.0053 g, 0.03 mmol) was dissolved in deoxygenated water (1 g). The AA solution was finally added to reduce Cu(II) and initiate the polymerization. Samples were taken during polymerization for analysis. Chain Extension. Chain extension experiments were carried out in THF solution. The emulsion samples were dissolved in THF and passed through a basic alumina column to remove catalyst residues. The mixture was participated in methanol, and PBMA-Cl macroinitiator was obtained after dried. Take run 4 as an example, PBMA-Cl (1.61 g, 0.1 mmol), MMA (1.5 g, 0.015 mol), dNbpy (0.0818 g, 0.2 mmol), and CuBr2 (0.02 g, 0.1 mmol) were dissolved in 8 mL of THF. The solution was then transferred to a flask and degassed by three

challenge lies in the design of ligand structure. Ideally, the ligand molecules are highly hydrophobic and stay in oil phase but do not mainly reside in monomer droplets and generate osmotic pressure that prevents the migration of monomer molecules from droplets to polymer particles. To meet this requirement, we introduce a novel design concept “surfactant− ligand” (SL); that is, a molecule that has both surfactant and ligand functionalities. These molecules form micelles and stabilize polymer particles. They also help to stabilize monomer droplets but do not dissolve in the droplets. The ligand functional groups of SL molecules reside inside micelles/ particles and could not come out to aqueous phase. It should be pointed out that monomer droplets also contain SL molecules. However, the droplets are much bigger than polymer particles. The molecules stay at droplet inner surface but do not mediate a significant level of polymerization inside the droplets. Scheme 1 shows the molecular structure of the SL design to be Scheme 1. Structure of the Surfactant−Ligand (SL) Design Synthesized and Used in This Work for ab Initio Emulsion ATRP

synthesized and used in his work. It is a block copolymer with hydrophilic poly(ethylene glycol) (PEG) and hydrophobic undecyl. The chain end of the hydrophobic block is functionalized with a multidentate ligand capable of complexing with copper ion.

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EXPERIMENTAL SECTION

Materials. Butyl methacrylate (BMA, Sigma-Aldrich) and methyl methacrylate (MMA, Sigma-Aldrich) were passed through columns of basic alumina (SCR) and then distilled under reduced pressure. Tetrahydrofuran (THF, SCR), dichloromethane (DMC, SCR), and isopropanol (IPA, SCR) were soaked with 4A molecular sieve (SCR). 2,2′-Azobis[2-(2-imidazolin-2-yl)propane] dihydrochloride (VA-044, 99%, J&K), ascorbic acid (AA, Aladdin), stannous octanoate (Sn(Oct)2, 95%, Sigma-Aldrich), 4,4-di(5-nonyl)-2,2′-bipyridine (dNbpy, 98%, TCI), Tween-80 (J&K), Brij-98 (TCI) and ethyl 2bromoisobutyrate (EBib, 98%, J&K), methoxypoly(ethylene glycol)s (mPEG, Mn = 1000, PDI = 1.02, Aladdin), 10-undecylenoyl chloride (99%, TCI), 3-chloroperbenzoic acid (mCPBA, 85%, Aladdin), copper chloride (CuCl2, 99.99%, Aladdin), copper bromide (CuBr2, 99.99%, Aladdin), triethylamine (TEA, SCR), N,N,N′,N′-tetraethyldiethylenetriamine (TEDETA, 90%, Sigma-Aldrich), and n-pentane (Aladdin) were used as received. Characterization. Monomer conversion was determined gravimetrically. Polymer molecular weight and distribution were determined by gel permeation chromatography (Waters e2695, Styrogel HR-3, HR-4, and HR-5) at T = 30 °C. THF was used as eluent with flow rate of 1 mL/min. Linear polystyrene standards were used for the universal calibration. Mark−Houwink constants were from ref 25. 1H NMR and 13C NMR spectra were recorded in CDCl3 or D2O as the solvent using a Bruker Advance 400 MHz spectrometer. Absorbance of Cu(II) catalyst solutions was measured using a UV−vis 7702

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Table 1. Emulsion ATRP of Butyl Methacrylate (BMA) with Surfactant−Ligand (SL) Catalyst Design run

SL/dNbpy (molar ratio)

time (min)

1 2 3 4

100:0 99:1 95:5 90:10

50 87 150 150

5 6 7

100:0 95:5 90:10

40 50 60

conv (%)

Mn,thc (kg/mol)

water-soluble initiator 4.60 4.20 4.30 3.50 oil-soluble initiatorb 99 28.4 96 27.2 90 25.5 99 89 90 75

Mn,exp (kg/mol)

PDI

dzd (nm)

PSDd

78.9 60.0 15.2 16.1

2.65 1.89 1.60 1.33

134 134 136 138

0.11 0.09 0.09 0.09

36.7 32.7 27.1

1.69 1.56 1.49

141 141 149

0.10 0.14 0.09

a

Experiment condition: 13 g of H2O, 1.42 g of BMA, 0.08 g of Brij-98, T = 80 °C, [BMA]/[VA-044]/[CuCl2]/[total ligand] = 100/1.5/1/1. Experiment condition: 13 g of H2O, 1.42 g of BMA, 0.08 g of Tween80, T = 70 °C, [BMA]/[EBib]/[CuCl2]/[total ligand]/[AA] = 200/1/1/1/ 0.6. cMn,th = ([BMA]/(2 × [VA-044])) × conversion × monomer MW for water-soluble initiator; Mn,th = ([BMA]/[EBib]) × conversion × monomer MW for oil-soluble initiator. ddz is intensity-average particle size, and PSD is particle size distribution index given by DLS. All latex samples were free of coagulum and stable at room temperature for more than 6 months. a b

Figure 1. Kinetic plot and number-average molecular weight and PDI as a function of monomer conversion for runs 1−4. Experimental conditions are given in Table 1. freeze−pump−thaw cycles. Sn(Oct)2 (0.03 g, 0.07 mmol) was dissolved in 0.1 mL of THF. The flask was placed in an oil bath at 80 °C, followed by Sn(Oct)2 addition. The polymerization was stopped after 12 h, and the mixture was participated in methanol. Determination of Cu(II) Content in Aqueous Phase. A calibration curve was generated for the CuCl2/Me6TREN catalyst complex in water. A series of solutions were prepared by varying the catalyst complex concentration (0, 0.05, 0.5, 2, 5, and 10 mM), and their UV−vis absorbances were measured. A good linear relationship between concentration and absorbance was obtained. After polymerization, the latex was centrifuged at 10000g for 30 min. The aqueous phase was transferred to a 15 mL ultrafiltration tube capable of removing molecules smaller than 3 kDa. Since the SL catalyst has a molecular weight of 1783 g/mol, it would go through the membrane if present in the aqueous phase. The ultrafiltration was conducted at 4000g for 30 min to ensure complete separation. Me6TREN was added to the separated aqueous phase at a ratio of CuCl2/Me6TREN (1/1.5), assuming 100% of the catalyst migrated to the water. Absorbance was measured using a UV−vis spectrometer at 800 nm. The absorbance values (runs 1−4) were 0.003, 0.015, 0.026, and 0.050, respectively, revealing that in the worst case (run 4) it was only 2 (see run 1 in Table 1). Figure 1A shows run 1 kinetics. There was an induction period, and the rate suddenly increased after the induction. The polymerization reached a high monomer conversion within 20 min. Evident from Figure 1B (SL/ dNbpy = 100:0), the polymer molecular weight was far from the theoretical line. There was no linear increase in molecular weight at the early stage. Run 1 was more or less similar to a conventional emulsion polymerization. In run 1, we used water-soluble initiator VA-044. The primary radicals were generated in water and entered into polymer particles. Some radicals were sacrificed in establishing equilibrium between Cu(II) and Cu(I) inside particles, causing the induction period. Other radicals propagated with monomer molecules and formed polymer chains. For good control of polymer molecular weight, these radicals must be frequently deactivated and activated during polymerization. The rate of deactivation must be orders of magnitude higher than that of activation. As a general rule, fast reactions such as radical deactivation could easily become diffusion controlled. With increase in particle size, radical centers become distant from particle wall, where catalyst complexes reside. Lack of molecular weight control in run 1 could be attributed to the diffusioncontrolled radical deactivation. Faucher et al.26,27 analyzed the molecular processes involved in ATRP mediated by supported catalysts and found that catalytic sites at the support surface effectively activated dormant chains but could not efficiently deactivate radical

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RESULTS AND DISCUSSION Emulsion ATRP with Water-Soluble Initiator. We employed the SL design in the emulsion ATRP of butyl methacrylate (BMA). More water-soluble CuCl2 was used as catalyst because it offered a better control than CuBr2. Watersoluble initiator VA-044 was first used. The latex was stable 7703

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reasons were believed to be twofold. First, the radicals were generated in water and experienced significant termination before entering micelles or particles. Second, for the radicals entering the oil phase, they must go through the particle wall, which was concentrated with SL catalyst. The radicals could be deactivated at the wall by Cu(II) and became small molecule dormant species with some returned to water and wasted. Emulsion ATRP with Oil-Soluble Initiator. On the basis of the hypothesis, we employed an oil-soluble initiator EBIB, which initiated polymerization in the oil phase. Since Cu(II) needed to be converted into Cu(I) before it could activate EBIB, we also added ascorbic acid (AA) as a reducing agent. AA diffused from water to oil and reduced Cu(II) into Cu(I) at the wall. Cu(I) then activated EBIB to initiate polymerization. Table 1 includes the results of emulsion ATRP with the oilsoluble initiator (runs 5−7). In run 5, 99% conversion was reached in 40 min. The polymer had experimental molecular weight close to its theoretical value. The initiator efficiency reached 0.78. However, PDI was 1.69, which was a little far from our satisfaction. Learning from runs 1−4, we added small amounts of small molecule ligand dNbpy. Runs 6 and 7 had some improved PDI’s from 1.69 to 1.56 and 1.49, with 5 and 10% of dNbpy (over the total amount of ligands). Figure 2 shows the polymerization kinetics, polymer molecular weight, and PDI of runs 5−7. It was evident that the polymerization had a first-order kinetics, and the polymer molecular weight increased linearly with conversion. Run 7 showed a very good agreement between the experimental M n,exp and their theoretical Mn,th, while the experimental data in runs 5 and 6 were higher than the theoretical values, which could probably be attributed to diffusion effects. Chain Extension and Livingness. We also investigated livingness of the resulting polymers. The polymer samples collected from runs 4 and 7 were purified and used as macroinitiators to prepare PBMA-b-PMMA block copolymers in solution ATRP. Figure 3 shows GPC curves of the chainextended samples. The chain extension experiments were clearly successful, with the GPC curves shifted to the high molecular weight end after chain extension. Some increase in the PDI was also observed, particularly evident in run 7. This could be attributed to higher conversion and faster polymerization rate in run 7, resulting in slightly more polymer chains losing functionality for extension. Evidence for Micellar Nucleation. To minimize the diffusion-controlled deactivation inside polymer particles, the free catalyst ligand dNbpy was added to improve the control

chains. It took too long time for the radical chains to diffuse to the catalytic sites for quick deactivation. It was actually a ppm amount of catalytic species leached from the support surface that effectively deactivated radical chains in the supported ATRP. Similar mechanisms could occur in run 1. The catalytic sites at particle inner wall could not efficiently deactivate radical chains in the bulk of particles. On the basis of this analysis, we added a small amount of small molecule ligand dNbpy. The dNbpy-ligated catalyst complexes could diffuse freely in the bulk of particles and facilitate the deactivation of radical chains. Scheme 2 illustrates this concept that SL-ligated Cu(I) at the surface activates dormant chains and dNbpy-ligated Cu(II) in the bulk deactivates radical chains. Scheme 2. Concept of the SL Catalyst Mediated Emulsion ATRP

In run 2, we added 1 mol % dNbpy (based on the total ligand). The polymerization rate slowed down, and the control of polymer molecular weight was slightly improved. Further increasing dNbpy to 5−10% (runs 3 and 4), significant improvements in the molecular weight control were achieved. The ln(M0/M) versus time relationships were linear, as shown in Figure 1A, and the polymer molecular weight increased with conversion, as shown in Figure 1B. The polydispersities of the final samples reached 1.60 and 1.33, respectively. We succeeded in improving the molecular weight control with the addition of small molecule ligand. However, the initiator efficiencies were very low. There were huge gaps between theoretical and experimental molecular weight data in Table 1. The initiation efficiencies were lower than 0.4. The

Figure 2. Kinetic plot and number-average molecular weight and PDI as a function of monomer conversion for runs 5−7. Experimental conditions are given in Table 1. 7704

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Figure 3. GPC results of polymers collected from runs 4 and 7 before and after chain extension. Chain extension experimental conditions were [MMA]/[PBMA-Cl]/[CuCl2]/[dNbpy]/[Sn(Oct)2] = 150/1/1/1/0.6, 8 mL of THF, T = 80 °C, and time = 12 h. (A) Run 4 and (B) run 7.

Figure 4. Change of polymer particle intensity size distribution during polymerization. (A) Run 4: an ab initio emulsion ATRP. (B) Control experiment: aqueous dispersed ATRP without the use of SL. Experiment conditions are 13 g of H2O, 1.42 g of BMA, 0.18 g of Brij-98, [BMA]/[VA044]/[CuCl2]/[dNbpy] = 100/1.5/1/1, and T = 80 °C.

with dNbpy only and without the use of SL. It was obvious in Figure 4B that the particle size changed slightly during the polymerization, suggesting that the droplets were directly converted into polymer particles and the monomer transfer did not occur this system. The highly hydrophobic ligand dNbpy generated the osmotic pressure between particles. The monomer could not effectively migrate from droplets to micelles/polymer particles, and the monomer droplets became the major nucleation sites. It became evident from these parallel experiments that the SL design in this work could effectively mediate the ab initio emulsion ATRP with micellar nucleation.

over molecular weight development in runs 2−4 and runs 6, 7. The micellar nucleation mechanism of the ab initio emulsion ATRP might become questionable. We therefore must prove that the mechanism was not affected with the addition of dNbpy in this work. The major difference between aqueous dispersed and ab initio emulsion polymerization lies in the role played by the monomer droplets. In an aqueous dispersed polymerization, monomer droplets act as microreactors, and they are the main nucleation sites. In contrast, in an ab initio emulsion polymerization, monomer molecules transfer from the monomer droplets to the micelles or polymer particles. The micelles are the main nucleation sites and the monomer droplets act as the monomer reservoirs. We measured the size change of polymer particles during the polymerization. Figure 4 shows the particle size distribution under two different conditions. The size distributions of emulsion prior to polymerization are given in Figure S1 of the Supporting Information. Micelles and monomer droplets coexisted in the monomer emulsion of run 4, while mainly monomer droplets were present in the control run. In run 4, the particle size increased from 73 nm in 20 min at 15% conversion to 150 nm in 120 min at 60% conversion, as shown in Figure 4A. The particle size (d) growth was clearly caused by the increase in conversion and the monomer migration from droplets to particles. A plot of d3 versus conversion is given in Figure S3 of the Supporting Information. A linear relationship between the d3 and conversion can be observed, suggesting the growth of particles and the number of particles remaining constant during the polymerization. The addition of 10 mol % dNbpy did not change the micellar nucleation mechanism. As control experiment, we carried out a run similar to run 4, but

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CONCLUSION In summary, we succeeded in an ab initio emulsion ATRP with micellar nucleation. The polymerization system showed good livingness and well control over polymer molecular weight development. Our approach was through the design of surfactant−ligand (SL). Such SL molecules acted as surfactant that stabilized polymer particles and as catalyst ligand that mediated ATRP. This design prevented highly hydrophobic ligands in monomer droplets, allowing the transport of monomer from droplets to micelles/particles. We also proved the addition of a few free catalyst ligands for a better control over the polymerization could not affect the mechanism of micellar nucleation in the ab initio emulsion ATRP.

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ASSOCIATED CONTENT

S Supporting Information *

Details about size distribution of the emulsions/latexes and latex particle volume change during the polymerization. This 7705

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(27) Faucher, S.; Zhu, S. P. Macromolecules 2006, 39, 4690.

material is available free of charge via the Internet at http:// pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected] (W.-J.W.). *E-mail: [email protected] (S.Z.). Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by the National Natural Science Foundation of China (Grants 20976153 and 20936006) and the Chinese State Key Laboratory of Chemical Engineering at Zhejiang University (Grants SKL-ChE-12T05 and SKL-ChE14D01).

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REFERENCES

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